1. Field of the Invention
This invention relates to compositions to be used in metal treating, specifically visually inspecting and cleaning metallic surfaces using a highlighting procedure or flux procedure, particularly iron or steel automotive structural elements.
2. Statement of Related Art
Metal surfaces made of iron or steel, for example, metal surfaces suitable for use in the production of automotive bodies, are subjected to a visual inspection prior to the phosphation procedure. By such inspection, locations of surface damage or material unevenness are detected, and parts having undesired quality features may thus be rejected. This is important since surface unevenness will not only affect the quality of the anticorrosive phosphated layer, but also cause the occurence of irregularities in the subsequent protective layers, for example in the varnish or lacquer.
For the visual inspection of metallic surfaces in the automotive industry there have so far been used hydrocarbon mixtures (gasoline, kerosine, etc.) and aqueous formulations containing large amounts of organic solvents and nitrite salts. The aqueous compositions containing such components are applied directly to the metal surfaces, forming a continuous liquid film which allows irregularities on the metal surface to be readily recognized. In this manner, a visual check of the iron or steel surfaces is facilitated. This procedure is generally referred to as "highlighting."
There is an important drawback in the solvent formulations used in the highlighting procedure in that workers engaged in checking the metal surfaces are continuously exposed to the organic solvent vapors. In addition, it is sometimes necessary for the workers to touch the metal surfaces so that their skin is in contact with the formulations, which creates another health risk due to the solvent proportions and the nitrite content. Furthermore, such components require a thorough and expensive cleaning of the waste water before it can be disposed of, as neither organic solvents (such as hydrocarbons), nor nitrites are allowed to be discharged with waste water. Additionally, films obtained by using highlighting formulations based on water/organic solvent/nitrite show an insufficient film coherence on some metal surfaces which makes the visual inspection difficult.
The inspection of metallic surfaces by means of the "flux procedure" utilizes the fact that iron filings become oriented in a magnetic field. A liquid containing fine iron filings is sprayed or poured onto a metal surface. Fissures in the metal surface are indicated in that the iron filings are irregularly orientated above the fissure location. The flux procedure usually employs aqueous formulations. The flux process is described in further detail in German Industrial Standard (DIN) 54,132. A drawback in the procedure is that mostly films having only a low coherence are formed, which do not allow a successful inspection of the metallic surfaces to be performed. Also, such formulations have proven to promote corrosion markedly, so that the quality of the coatings obtained by the subsequent application of coating materials is significantly affected. Moreover, the visual surface inspection is only one of several stages of preparation for the actual coating process. In the above conventional procedures it happens time and again that materials are introduced which promote foaming or other undesired side-effects.